Multimedia terminals for processing two or more multimedia signals and either storing them or sending them over communications lines to other terminals are presently evolving. Such terminals require not only circuits to receive and transmit analog video signals and audio signals but they also require circuits which digitize and compress the data and package it for transmission so that it may be received at a receiving station, decoded and the original analog signals regenerated. Because of the speed required to transmit the high volume of data involved, efficient means of packaging the data for transport is essential. Adding to the difficulties is the fact that source signals may be stable or unstable. For certain applications such as video teleconferencing a minimum delay is important whereas for other applications where the source signal is not stable, it may be possible to sacrifice latency and accept loss of signal in order to achieve a continuous signal.
A sending end clock sends out analog data at a sending clock rate. This data is received after transmission by a multimedia terminal and digitized. The digitization is accomplished by regular sampling of the analog signal and generation of a digital word to represent each sampled signal. In addition, control information is extracted from the headers of the analog signal after digitization. If one extracts the clock of the incoming analog signal, then a minimum delay is required to package the data and ready it for transport. However, should the signal be unstable then an undesirable affect on the received signal results. Often it is possible to sacrifice delay or latency for signal quality. In other cases it is important to minimize delay.
Accordingly, it is an object of the present invention to provide an improved multimedia terminal which can accommodate a requirement for minimum delay and also accommodate unstable signals where delay is not of crucial importance.
One of the main requirements of an encoder, in particular the Motion Pictures Experts Group-2 (MPEG-2) encoder, is that the digital data received by the encoder must be synchronized to the system time clock since the system time clock (STC) signal is used for timing in the encoder. The system time clock signal is periodically sampled and as part of the multiplexing (mux) operations is placed on a transport data stream. Previous means of synchronizing the STC signal with the sending clock rate of the analog signal involved locking the system clock to the analog signal and using a phased lock loop to generate a specific rate.
The disadvantage with this type of synchronization is that an interruption of the input signal or an unstable data signal results in jitter. Thus, there is a need for an alternative method that synchronizes the video data with STC without being subject to disruption of the analog signal.
In addition, under MPEG-2, there is a requirement that the encoder receive only whole video frames. Consequently, there is a need for a mechanism which ensures that the encoder receives only whole frames of data.
Accordingly, it is an object of this invention to provide an improved frame buffer which can operate in two modes. One of the modes synchronizes the asynchronous field rate with the STC while the second mode passes data through with minimal delay.